igicrf VESTAL— PFIYTOGEOGRA PHY OF COLORADO 



TABLE VI 



Effects of direction of slope upon local distribution 



185 



Factors other than physical conditions of habitat. — 

 If the physical conditions which determine the habitat and all 

 their interactions and variations were fully known, however, the 

 local distribution of plant communities as observed would only 

 partially be explained. Within even a very small part of the 

 region studied correlations between physical habitats and 

 vegetation-types must not be too closely drawn. The rock pine, 

 for example, grows in any soil or on any slope; its presence or 

 absence in any particular situation is not alone a matter of physical 

 conditions there and then operative. Local distribution of 

 vegetation-types in these partly unstable and locally very diverse 

 situations depends also on at least three other sets of conditions: 

 (1) range of toleration, in individual species or groups of species, 

 of variation of physical conditions; (2) local historic factors, physi- 

 cal and vegetational, which have been operative in any given spot 

 (these often cannot be determined); (3) accident of seed 

 distribution and germination. For these reasons it seems best to 

 characterize the vegetation-units, in most cases, from the vegeta- 

 tion itself rather than from habitat. There can be no question 



